This invention relates to a method and apparatus for thermal homogenisation of molten metal in a metallurgical vessel and in particular but not exclusively to electric arc furnaces for steel production.
Electric arc furnaces in steel processing are used to melt down ferrous materials and refine them into molten metal before tapping into a ladle. The furnaces usually comprise a hearth of generally crucible shape into which ferrous materials such as, for example, scrap, are charged, one or more electrodes adapted to melt the ferrous material with electric arc heating to form a bath of metal and one or more gas injection ports in the bottom of the hearth through which argon is directed into the molten metal for the purposes of stirring the bath.
To enhance melting of ferrous material and to post combust carbon monoxide within the furnace it has also been proposed to blow oxygen into the hearth from above the level of the bath.
In more recent times, other hearth shapes have been proposed. Such hearths generally comprise a deep portion, referred to hereinafter as a crucible, adapted to hold the bulk of the molten metal, at one end of the hearth and at the other end a shallow portion, hereinafter referred to as a balcony. The balcony is generally provided with a taphole in its base or an outwardly projecting spout in its wall. The one or more electrodes used in the furnace are generally placed within the crucible zone. Typically hearths of this kind are tiltable to facilitate tapping.
Additionally, they are so shaped to enable some molten metal to remain in the crucible at the conclusion of the tilting/tapping operation to avoid slag entrainment when tapping the steel.
A problem that has been observed in electric arc furnaces in general is that the temperature of the molten metal in the hearth is not homogeneous. In particular, "cold spots" and "hot spots" have been observed in the hearth, that is, positions in the hearth at which the temperature of the molten metal is relatively lower and higher respectively than that of the bulk of the molten metal.
It has been observed that this disparity is exacerbated in hearths having balconies and that the temperature of the molten metal in the balcony is relatively lower than that of the molten metal in the crucible. It has also been observed that the temperature of the molten metal towards the walls of the hearth is relatively lower than that of the molten metal towards the centre of the crucible with the exception of the areas near the electrode where the opposite is the case.
It has also been observed in hearths provided with means to blow oxygen into the hearth from above the level of the bath that continued blowing of oxygen after the scrap has melted can produce splashing of molten metal onto the walls of the hearth and back splash onto the oxygen blowing means causing deterioration of the wall surfaces and of the oxygen blowing means respectively.
Continued operation of the scrap melting apparatus after the ferrous materials have been converted to molten metal is often required to heat the molten metal to a predetermined temperature and/or maintain that temperature prior to tapping.
An object of the present invention is to address the above problems of the prior art.